# Photonic force?

1. Sep 27, 2004

### Phymath

while im most likely completely wrong I've got an idea...
while we all know force is...
F = dp/dt

and the momentum of a photon is p = hf/c
then wouldn't you create an acceleration if u had a flashlight that changed frequence(color) extremely quickly, yes i know the momentum would be so small that you wouldn't experience it but shouldn't it still happen

dp/dt = h/c df/dt

a recoil from the frequencey change or a pulsing light would also work, from f = 0 to f = some color frequencey.

ma = h/c df/dt
$$a = \frac{h}{cm} \ \frac{df}{dt} \ \frac{1}{m}$$

2. Sep 27, 2004

You seem to have missed the point that as you change colour quickly, all you are doing is emmitting photons of different colours at different times. How does this create a change in momentum?

3. Sep 27, 2004

### Phymath

yes i see what u mean, but if the photon did have a frequency shift would there be an acceleration? (next question: how would u make a photons frequency shift? dopper effect? would that change the orginal problem?)

4. Sep 27, 2004

### HallsofIvy

Don's ya just hate it when someo one shines a flashlight on you and it nocks you back several feet?

5. Sep 27, 2004

### HallsofIvy

Don's ya just hate it when someone shines a flashlight on you and it nocks you back several feet?

6. Sep 27, 2004

### reilly

Classical theory is quite suffficient to indicate no acceleration for the photon in vacuum. From Poynting's Thrm it's clear that any change in momentum, field + particles, is due to moving, accelerating charges, which, of course, experience forces. The field changes due to the motion of the charges -- emitting and absorbing photons

I suspect that it might be possible to "accelerate" a wave packet in a dispersive medium. Interesting problem.

Photons in vacuum do not accelerate. There's nothing to push or pull,
Regards,
Reilly Atkinson

7. Sep 27, 2004

### Phymath

while i don't really what ur trying to say about changes in momentum, other than i think your trying to say that momentum only changes with particles with mass...but anyways the point i was making is that the photons didn't need to "accelerate" to create a force, photons have not been discovered to have mass currently, but still they have momentum...

8. Sep 27, 2004

### pervect

Staff Emeritus
I don't quite get why you want to change the photons frequency. A flashlight will already produce a tiny amount of thrust. It's a very low power example of a "photon drive", a rocket where the exhaust is photons. The frequency of the photons in a photon drive does not matter - the momentum in the exhaust P is the energy of the exhaust E divided by the speed of light

p = E/c

irregardless of the frequency of the photons.

9. Sep 28, 2004

### naunzer

But the energy of a photon depends on its frequency by
E=hf,
so we are finally back at the starting relation p=hf/c.

10. Sep 28, 2004

### Gonzolo

Keeping the same intensity (= number of photons), a change in frequency should correspond to a change in momentum, simply because :

p = hf/c

where p is the momentum of the photon, and -p that of the emitter. If you have a blue laser and a red laser of equal intensity, the blue laser should provide more recoil than the red. Finely tuning the frequency from red to blue from a single laser however is more problematic.

11. Sep 28, 2004

### reilly

Momentum and Poynting's Thrm

Yes, photons have momentum because the E&M field has momentum. That this is true is one of the more profound results of classical physics. Further, a key result is that the momentum of charged particles added to the field momentum is conserved. There are some subtle points in the arguments, some of which deal with whether or not there is an ether. Many E&M and Relativity texts cover this material in great detail. Check it out.

Also, the structure of QED diagrams illustrate the profound difference between photons and charged particles. Charge lines go on forever -- loops are forever; photon lines end all over the place -- creation and destruction and all that.

Regards,
Reilly Atkinson

12. Sep 28, 2004

### Gonzolo

If you change the frequency of photons that are already emitted, the flashlight will not know it, it couldn't care less. The flashlight's momentum was given to it at the instant of emission. Whatever happens to photons afterwards is irrelevant to this problem.